<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>边框制作三角形</title>
</head>
<style>
    div{
        box-sizing: border-box;
        border-top: 50px solid #e656e6;
        border-left: 50px solid transparent;
        border-right: 50px solid transparent;
        border-bottom: 50px solid transparent;
        width: 50px;
        height: 50px;
        transform-origin:  center 25%;
        transform: rotate(180deg);
    }
</style>
<body>
    <div></div>
</body>
</html>